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Closing In on a Giant Black Hole

A new analysis of x-rays streaming from the bright center of a galaxy has strengthened the case that a supermassive black hole lurks there. The x-rays imply that their source, a spinning disk of gas, surrounds an object with a density only a black hole could reach. The research, reported in tomorrow's Nature, represents the most unambiguous scrutiny to date of the bizarre environment close to a large black hole's maw and supports claims of supermassive black holes in other galaxies.

The Hubble Space Telescope has already found signs that enormous black holes dwell in the centers of some galaxies by spying stars swirling near their cores, as if around powerful gravitational drains. Two years ago, researchers used x-ray telescopes aboard a Japanese satellite called ASCA to gain an even more direct view. The satellite detected radiation from glowing iron atoms moving at fantastic speeds--at least 20% of the speed of light--within a flattened doughnut of gas at the core of a galaxy called MCG-6-30-15. The doughnut seemed to be centered on a fantastic concentration of mass. But to distinguish between a dense clump of stars and a black hole, the team needed to know the disk's size. Their size estimate depended heavily on questionable assumptions about the nature of the disk itself.

Theorist Benjamin Bromley of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleagues have now removed those uncertainties. The team examined the iron spectrum from MCG-6-30-15 with a different analytical technique that could directly calculate the distance of the iron atoms from the center of disk--regardless of the shape and other properties of the disk. The rapidly swirling inner region of the disk, they deduced, is at most 2.6 times bigger than a 10 million solar mass black hole would be. Because of this close match, says Bromley, "we absolutely have to abandon any model other than a supermassive black hole."

Astrophysicist Shuang Zhang of NASA's Marshall Space Flight Center in Huntsville, Alabama, agrees. Further, he notes, the x-rays also offer a tantalizing first glimpse of the black hole's possible spin--estimated by Bromley's team at about one-quarter the maximum rate allowed by Einstein's general theory of relativity. "Black holes can exhibit only three properties: mass, spin, and charge," Zhang says. "To constrain two of those three for a supermassive black hole is very exciting."